Force limiting devices are in use, especially in vehicle safety systems, and make it possible to absorb a belt withdrawal force occurring in the vehicle safety system in the event of a crash. The force limiting device enables a limited belt discharge relative to a component holding the belt. These types of force limiting devices are used both in vehicle safety systems that operate rotationally in the form of preferably self-locking seatbelt retractors and also seatbelt retractors combined with tensioners or in the form of pure tensioning devices, such as in the form of end-fitting tensioners, as well as in vehicle safety systems that operate linearly, in which the belt is held on a fitting piece, which moves linearly, by means of a piston/cylinder arrangement for example, either for tensioning or for adjusting a controlled belt discharge.
It is desirable, however, to also integrate a force limiting device at other locations in a motor vehicle in order to use relative motions, brought about by a crash, between two parts of the motor vehicle for energy dissipation, for example, between a bumper and the vehicle frame.
A force limiting device provided in a seatbelt retractor as part of a vehicle safety system is known from EP 1 222 097 B1 for example. In this case, the force limiting device consists of a torsion bar, one end of which is connected to the belt shaft and the other end of which can be fixed to the housing by means of a suitable locking device. If the seat-belted vehicle occupant is displaced forward in the event of an accident because of the vehicle acceleration or deceleration that occurs, resulting in a corresponding belt withdrawal force acting on the belt shaft, then the belt shaft can rotate by a certain amount, with the torsion bar twisting at the same time, so that a belt discharge will occur over a corresponding rotary travel of the belt shaft. The force that restrains the occupant in his/her forward movement is thereby absorbed. The known belt retractor provides for a second force limiting device in the form of an inertial mass which can be coupled to the belt shaft, the effect of the inertial mass being superimposed on the responding effect of the torsion bar. Depending on the speed at which the seat-belted person moves forward and also depending on the occupant data of the seat-belted person, the inertia of the inertial mass which has been put into rotation by the belt shaft becomes effective such that an additional energy-dissipating and/or force-limiting component becomes effective.
However, the known seatbelt retractor and its force limiting device has the disadvantage that the effect of the force limiting device depends on the occupant, particularly on the occupant's size, weight and seat position in the vehicle, as well as on the severity of the accident and thus, on the momentum conveyed to the occupant in an accident and on the resulting acceleration forces. The effect of the force limiting device therefore results in different force levels, which can be adjusted to the seat-belted occupants and controlled only at great expense in order to prevent the seatbelt system from placing too great a strain on the occupants during an accident.